Once you have defeated the Poketopia Master (Colloseum Leader of the Stargazer Colloseum), you will unlock a secret gift under the "Mystery Gifts" part of the shop. You can download this to Pokemon Diamond or Pearl. It is free, and the gift is a level 10 Pikachu that knows Volt Tackle, Surf, Tail Whip, and Thunder Wave.

Pokemon XD reference

At Sunset Colosseum, take a look around before a battle. It is a replica of the Orre Colosseum from Pokemon XD.